Answer:
Step-by-step explanation:
The position equation is given as
[tex]s(t)=-16t^2+21t+3[/tex] and we are looking for the times when the position of the ball is 9 feet. That means that we simply plug in a 9 for s(t) and factor to solve for t:
[tex]9=-16t^2+2t+3[/tex] and
[tex]0=-16t^2+21t-6[/tex] and this is what we factor to find t:
t = .42 sec and then again on its way back down at t = .89 sec
